What about things like Murkrow, Houndour, and Misdreavus in GSC and HGSS (Houndour specifically)? Those are available before Red, but not for the primary game. I realize you could still do the challenge with them, but it would be significantly harder to wait for them until you've beaten the vast majority of the game.

I didn't include them precisely for that reason, as the challenge doesn't end until Red. I know Houndour is the odd man out here, as Misdreavus and Murkrow are both available in the HGSS Safari Zone while you are still in Johto, but the Johto games have always been a bit abnormal in comparison to the rest on the main series games.